Vibratory compactors having drums are used for compaction of soil, asphalt, and other ground surface materials. The compactor is a self-propelled vehicle having at least one drum equipped with one or more eccentric weights mounted on a power-driven shaft. The speed of rotation of the shaft is varied to obtain varying dynamic forces which impart vibrations to the drum. The frequency and amplitude of the vibrations imparted to the drum have been varied by providing a liquid weight to a resevoir rotated relative to the drum. The amount of liquid stored in the resevoir is used to vary the eccentric weight within the shaft. This vibratory system is disclosed by Boone in U.S. Pat. No. 3,656,419. A vibratory mechanism for a drum that does not have an external liquid resevoir is disclosed by Boone et al in U.S. Pat. No. 3,616,730. This vibratory mechanism has a pair of resevoirs connected on opposite sides of a power driven shaft. The fluid can flow between the resevoirs so as to selectively balance or vibrate the drum at a desired amplitude. Eccentric weights controlled by pistons to selectively produce either high or low amplitude vibration of a drum are disclosed by Barrett et al in U.S. Pat. No. 3,814, 532. An eccentric mass is movably mounted on a carrier for rotation about an axis parallel to the drum face. A releasable locking device, such as a hydraulic jack mounted on the carrier controls the position of the movable mass. A second eccentric mass fixed to the carrier is selectively counter-balanced by the movable mass thereby control the vibration amplitude imparted to the drum. A dual amplitude, rotational vibratory mechanism for a compaction drum in which the amplitude of the vibration can be changed by reversing the direction of rotation of the mechanism as disclosed by Stanton in U.S. Pat. No. 4,586,847. These vibratory mechanism are not variable between a balanced mode and a maximum vibration mode.